This invention relates to a process for manufacturing a novel inorganic heat insulating material, more particularly to a novel, useful process for stably yielding, in a short period of time, an inorganic heat insulating material containing uniform foams, which foams and hardens simply by mixing up the ingredients into pasty state, even without any heating operation.
There have conventionally been various proposals for obtaining inorganic foam materials based upon aqueous solution of alkali silicate. Among such known processes there are, for example, a process for foaming the solution by directly heating same, a process of first mixing to the solution a foaming agent as will give rise a gas upon heating and then getting the mixture to gel followed ultimately by foaming up the mixture gel by means of heating same, and a process of first mixing to the solution a hardening agent such for instance as silicofluoride followed by heating the mixture thus to hardening and foaming same. All such known processes essentially require heating (normally in the range of 200.degree.-900.degree. C.) for obtaining the foamed material. Namely, the alkali silicates and the foaming agents never cause foaming reaction at the normal temperature, and heating is indispensable for the foaming. It is yet more to be noted that the foamed product obtained by any of such processes contains water-soluble alkali components as will easily dissolve out upon contact with water thus for markedly impairing the structural strength of the foamed product, which has, therefore, very narrow scope of application as the heat insulating material, becuase of such low resistance to water by nature.
In the field of concrete and mortar, inorganic lightweight materials highly resistant to water and with high mechanical strength are conventionally known, for instance as lightweight concrete, lightweight mortar or the like, but most of them are made simply by incorporating proper lightweight aggregate such as perlite, vermiculite or the like. It is also a known process to mix metal aluminum and water with cement, to knead up the mixture and to submit the mixture under heat and pressure in an autoclave, thus for causing exothermic hydraulic reaction with simultaneous foaming by hydrogen gas generation, but this process requires troublesome operations as curing in the autoclave, and the time required for the foaming and hardening is very long, particularly the hardening normally requiring quite long as one whole week or so. It should further be noted that the various processes as mentioned above can hardly provide the foamed product sufficiently light in weight, the best lightweight conventional product having the density of more than 0.5 specific gravity.
The present invention has as its object to provide a novel process for manufacturing a useful inorganic heat insulating foamed material eliminating all the drawbacks of the conventional processes for manufacturing such material.
In order to attain the object, the process for manufacturing the inorganic heat insulating material according to this invention is characterized by mixing up into a pasty state, under presence of water, the ingredients comprising: (A) water-soluble alkali silicate (hereinafter referred to as ingredient A); (B) alumina cement (hereinafter referred to as ingredient B); (C) metal base foaming agent (hereinafter referred to as ingredient C); and (D) foam stabilizing agent (hereinafter referred to as ingredient D).
One of the most prominent features of this invention is to easily yield the desired heat insulating material under normal temperature and normal pressure simply by mixing up the said ingredients A-D into pasty state, even without any heating operation subsequent to the mixing. To note further, foaming reaction of the said mixture requires only short period of time, normally in the range of 5-60 minutes, which is defined almost definitely by the composition of the mixture, and subsequent hardening proceeds also rapidly, normally to complete within 24 hours. Furthermore, application of the mixture in pasty state as mentioned above allows use of the casing mould or frame almost in any complicated shape without causing difficulty, thus enabling to easily form the product in any design as desired. Foaming pressure of the paste is rather low, as will permit to use even corrugated paper board for the casting wall, thus requiring no specific casting frame of substantial strength, and the paste can be applied by pouring into the place desired to be heat insulated, simply with proper confinement walls. The pasty mixture according to this invention is further characterized by the excellent stability of the foaming reaction as is little influenced by the ambient conditions as climate or the like, and it provides the possibility of regulating the foaming reaction time, by properly regulating the composition ratio of the said ingedients A-D, which may thus be set and then kept almost uniform and constant to the desired value within the said possible range, and also of easily regulating the foaming overrun ratio, thus the bulk density of the product. With respect to the bulk density, in particular, extremely low density can hereby be provided, as in the range of about 0.1-0.3 g/cm.sup.3 as has never been possible with respect to the conventional autclaved lightweight concrete, generally called ALC and known as with excellent mechanical strength; with this novel product of such low density still having sufficient mechanical strength for practical use as the heat insulating material. As a matter of course, here is no difficulty in manufacturing the product with similar bulk density and similar mechanical strength just as the said ALC, and such novel product can now be of half as low heat conductivity coefficient as compared with the ALC.
The inorganic heat insulating material provided by the process of this invention has the foams of substantially uniform diameters, in the possible range of 0.5-10 mm as the case may be, and the foam structure is very robust. This material has thus excellent heat insulation, noncombustibility, resistance to heat, and interception of fire flame. Especially, the heat resistance is quite excellent as is proved by the test of keeping the samples in a 700.degree. C. furnace for 24 hours, resulting no appreciable deformation of the samples at all. Still more, this material according to this invention has remarkably excellent resistance to water, acid and alkali, as well as the mechanical strength, as are not realized by the conventional foamed alkali silicate material.
The reason why the novel inorganic heat-insulating material with the properties as mentioned above can be manufactured according to the process of this invention simply by mixing up the said ingedients A-D is not very clear as at this moment, but it might perhaps be as follows: Upon mixing up into the paste, most of the ingredient B, namely alumina cement (or the same together with portland cement), reacts upon water to gradually be hardened as hydraulic reaction, while a part of the ingredient A, namely water-soluble alkali silicate, as well as of the said ingredient B, undergoes hydrolysis in the paste to give alkaline agents, such as alkali hydroxides, and the groups such as SiO.sub.3.sup.2- and AlO.sub.2.sup.-, with said alkaline agents then coacting with the ingredient C, namely metal base foaming agent, to promote the foaming action for generating the minute foams within the paste or the hardening ingredient B, and with said groups such as SiO.sub.3.sup.2- gradually undergoing gellation in parallel with said foaming reaction, for being intimately packed up within the mass of the hardened ingredient B, thus resulting, according to this assumption, in improved mechanical strength of the foamed product as solidified. As for the ingredient D, namely the foam stabilizing agent, it is assumed that it will, while the hydraulic reaction of the ingredient B and the foaming reaction between the ingredients A/C and B/C proceed, keep the dispersion of the ingredient C uniform within the entire bulk in spite of its inclination to otherwise gradually be locally biased, thus securing the function for stabilizing the foaming reaction and preventing localization as well as serial continuation of the minute foams as generated. Be the matter what it may, this invention enables to stably and easily manufacture the inorganic heat insulating material with excellent characteristics, simply by mixing up the ingredients uniformly, under normal temperature and normal pressure, thus providing enormous industrial value.